1. Field of the Invention
The present invention relates to an image forming apparatus, such as a laser beam printer (LBP) and a copier, in which an electrophotographic process and the like are used.
2. Description of the Related Art
With recent advances in computer network technologies, printers serving as image output terminals have rapidly become widespread. In recent years, as the color image output has been developed, there has been an increasing demand for improved stability of the image quality of color printers and for greater uniformity of the color image quality among different color printers.
In particular, there has been a demand for higher stability in the reproducibility of colors and in the accuracy of superimposing colors, irrespective of a change in an installation environment, a secular change, or a machine difference. With an electrophotographic image forming apparatus, however, image density or color registration varies due to a change in an environmental condition in which the image forming apparatus is placed, a deterioration of a photosensitive member or a developer over time, or a change in the temperature inside the image forming apparatus, and thus it is difficult to meet such a high demand without modifying the initial settings.
Thus, an optical sensor is typically used as a toner detection device that carries out feedback control for maintaining the image density and the color registration in optimum states. Such feedback control is carried out in the following manner. A test toner image (hereinafter, referred to as a “test pattern”) is formed on a circulatory moving body, such as a photosensitive member, an intermediate transfer member, and a transfer conveyance belt, and the density of the test pattern and relative positions of the colors are measured by the optical sensor serving as the toner detection device.
Based on the measurement result and the conditions under which the test pattern has been formed, the image density and the color registration are then controlled so that the image density and the color registration would be optimized in actual printing. Parameters to be controlled include, for example, an exposure pattern in forming a latent image, an exposure start position, an image forming magnification, a developing bias, a charging bias, and the like.
As such an optical sensor serving as a toner detection device (toner detection sensor), a sensor that radiates light onto a test pattern and optically measures a toner amount or the position of a toner image based on the reflected light is often used.
Japanese Patent Application Laid-Open No. 2006-267644 discusses an optical sensor that includes, as optical elements, a light emitting element (light emitting diode (LED)) that radiates light onto an irradiated surface of a measurement target, a light receiving element for receiving specular reflection light, and a light receiving element for receiving diffuse reflection light. Each of the light emitting element and the light receiving elements is a so-called shell-type optical element, and is provided with a semiconductor chip including a light emitting portion or a light receiving portion, a shell-type lens unit, and a lead frame to be connected to a circuit board. With such a shell-type optical element, the orientation of the optical element can be modified freely to a certain degree by changing the angle at which the lead frame is bent. Therefore, each of the optical elements can be oriented in a desired direction by fitting each of the optical elements into a housing. However, a shell-type optical element may include a lens unit or a lead frame that is long to a certain degree so that the orientation of an element can be changed, and thus a certain volume is required between the semiconductor chip and the circuit board, which leads to a disadvantage in terms of downsizing the sensor.
In the meantime, in order to downsize a sensor, Japanese Patent Application Laid-Open No. 2006-208266 discusses an optical sensor in which an optical element, which is a chip component to be mounted on the surface of a circuit board, is used and the circuit board is covered by a housing provided with a light guide path. When an optical element that is to be mounted directly on the surface (mounting surface) of a circuit board is used, a lead frame and a lens unit are not provided, and thus the volume necessary for mounting the optical element directly on the circuit board is greatly reduced, enabling downsizing of the sensor. Typically, a chip-type light emitting element does not include a lens unit that is integrally formed with a light emitting portion as in a shell-type light emitting element, and thus has a large directional angle light emission. Therefore, with the configuration discussed in Japanese Patent Application Laid-Open No. 2006-208266, light from the light emitting element is condensed by using a condensing lens, and an irradiated surface is irradiated with the condensed light. However, with an optical sensor in which a chip-type optical element is mounted on the surface of a circuit board as discussed in Japanese Patent Application Laid-Open No. 2006-208266, a variation is likely to occur in a position at which a light emitting element is mounted on the surface of the circuit board. Therefore, in a case in which the position of the light emitting element is not aligned with the optical axis of the condensing lens, an irradiation position on the irradiated surface is likely to vary, and light having desired light amount cannot be radiated at a desired position on the irradiated surface. Therefore, the amount of light received by a light receiving element decreases, and the detection accuracy may be degraded.
Thus, Japanese Patent Application Laid-Open No. 2013-191835 discusses a configuration in which, without using a lens, an irradiated surface is irradiated with light from a light emitting element through an aperture provided in a housing that covers the light emitting element. According to this configuration, even if a variation occurs in the position at which the light emitting element is mounted, an irradiation position on the irradiated surface is less likely to vary.
However, with the configuration in which light is radiated from the light emitting element through the aperture, stray light may be generated depending on the shape of the housing. In other words, for example, some of the light emitted from the light emitting element is reflected inside the housing prior to being emitted through the aperture, and passes through the aperture in a direction that is different from an originally intended direction. Thus, the light is radiated onto the irradiated surface at a position different from a desired position and specularly-reflected, and the reflected light reaches, as stray light, the light receiving element that receives diffuse reflection light. In such a case, the stray light may change the amount of light received by the light receiving element that receives diffuse reflection light, and a correct output may not be obtained from the light receiving element. Consequently, the accuracy in detecting the irradiated surface by using an optical detection device may be degraded.